The 1st Law of Thermodynamics is used to derive a new climate change equation. This is then used with graphed data - of a composite solar irradiance record and from the International Cloud Climatology Project (ISCCP-FD) and NASA’s – to examine the energy changes that resulted in global warming between 1990 and 2000. Finally, graphed data from NASA’s Clouds and Earth’s Radiant Energy System (CERES) instrument on the Terra satellite is used to look at the question of planetary warming or cooling in the last decade.
All planetary warming or cooling in any period occurs because there is a difference between incoming and outgoing energy, an energy imbalance. The imbalance results in changes to the amount of energy stored, mostly as heat in the atmosphere and oceans, in Earth’s climate system. If more energy enters the atmosphere from the Sun than is reradiated back out into space – the planet warms. Conversely, if less energy enters the atmosphere than leaves – the planet cools. Thus Earth’s energy budget can be completely defined in three terms. In any period, energy in is equal to energy out plus the change in the amount of stored energy.
This can be expressed as:
EIN/s - EOUT/s = d(GES)/dt.
By the law of conservation of energy - the average unit energy in (EIN/s) at the top of atmosphere (TOA) in a period less the average unit energy out (EOUT/s) is equal to the rate of change (d(GES)/dt) in global energy storage (GES). The most commonly used unit of energy is Joules. Energy in and energy out is most commonly reported in Watts (or Watts/m2) – and is more properly understood to be a radiative flux or a flow of energy. A flux of one Watt for one second is one Joule – which is known as unit energy. Most of the stored energy is stored as heat in the oceans which is measured in Joules (or Joules/ m2).
Energy in as visible (shortwave) and infrared (longwave) radiation varies marginally over the 11 year Schwabe solar cycle, perhaps a little more over the longer term due to solar variation and, due to orbital changes, over an Ice Age. In the period 1990 to 2000 (from peak to peak of the solar cycle) – there was no overall trend in energy in.
Figure 1: Composite Solar Irradiance Record (after Lean 2010)
Most change in the global energy imbalance occurs in energy out. Energy out is measured by satellite in the visible and infrared spectrums. As opposed to the absolute values given for incoming radiative flux (which should be considered to be approximate) the data on outgoing radiative flux is commonly reported as radiative flux anomalies with a nominal zero point. Only the trend is of any relevance.
Increasing greenhouse gases in a period absorb more outgoing longwave radiation and energy out decreases thus creating an energy imbalance which results in an increase in energy stored (mostly as heat) in the climate system. Reflected shortwave radiative flux is influenced by land clearing, ice, volcanos, dust and, importantly for short term changes, changes in cloud cover. Again, these changes cause an energy imbalance which results in changes to the heat content of the atmosphere and oceans.
Graphed data the NASA/GISS International Satellite Cloud Climatology Project (ISCCP-FD 2007 analysis) are shown below in Figures 2, 3 and 4 showing shortwave up, longwave up and net radiative flux respectively.
Neglecting the big spike in 1992, which was caused by sulphur emissions from the Mt Pinatubo eruption, the trend in Figure 2 is to quite substantially less reflected shortwave radiative flux over the period 1990 to 2000. Overall the graph shows a "slow decrease of upwelling SW flux from the mid-1980's until the end of the 1990's and subsequent increase from 2000 onwards...." that appears to be caused "primarily, by changes in global cloud cover (although there is a small increase of cloud optical thickness after 2000) and is confirmed by the ERBS measurements." (http://isccp.giss.nasa.gov/projects/browse_fc.html) The trend was to less energy leaving the climate system between 1990 and 2000 and, other things being equal, this must result in more energy being stored in the atmosphere and oceans.
Figure 2: ISCCP-FD Shortwave Radiative Flux Up Anomaly
The infrared radiative flux up in Figure 3 shows a striking increase over the period. Again, this result reflects changes in cloud as less cloud allowed more infrared radiation to escape into space. This is a period where anthropogenic greenhouse gas emissions were expected to result in a reduction – by 0.45W/m2/per decade - of infrared emissions and therefore a positive energy imbalance and planetary warming. Without a doubt, anthropogenic greenhouse gases influenced planetary infrared emissions, however, it is not possible to distinguish the anthropogenic signal in the satellite record from the noise of climate variation.
Figure 3: ISCCP-FD Longwave Radiative Flux Up Anomaly
The trend of net radiative flux (shortwave plus longwave) is, by convention, shown as energy gained or lost by the planet. A trend upward is energy gained and, conversely, a trend down shows energy lost. Thus the upward trend in net radiative flux anomalies between 1990 and 2000 in Figure 4 (again ignoring the Mt Pinatubo spike) shows that less energy was leaving the Earth’s climate system at the end of the period than at the beginning.
Figure 4: ISCCP-FD Net Radiative (LW + SW) Anomaly
The analysis is simple – as a result of the period chosen of 1990 to 2000. We know the planet did warm in the period in both oceans and atmosphere. Thus the change in the global energy storage term was positive and this must, by a fundamental law of physics, be caused by an energy imbalance. Therefore EIN/s was necessarily greater than EOUT/s in the period. As there was no trend in incoming energy - the cause of planetary warming was in less energy out, seemingly exclusively as less reflected visible light leaving the planet.
Later radiative flux data from NASA’s Clouds and Earth’s Radiant Energy System (CERES) instrument on the Terra satellite is available.
Figure 5: CERES Net Radiative Flux Anomaly (after Dessler, 2010)
The CERES record does show large changes in radiative flux that are associated with changes in the state of the El Niño Southern Oscillation (ENSO) and related cloud changes (Dessler 2010). There was, however, no obvious simple trend of any note over the short period of the record – with recent cooling offsetting earlier warming. Combined with the decline in energy in as solar irradiance fell to a minimum in 2008 – there is a suggestion that the planet cooled (net ocean and atmosphere) a little.
The question arises of whether the large decrease in reflected sunlight, and the anomalous increase in infrared emissions, in the period from the mid 1980’s to the late 1990’s can be seen as artefacts - cloud feedbacks - of anthropogenic global warming. Observational evidence suggests a negative correlation of cloud with sea surface temperature in ENSO states (Dessler 2010, Zhu et al 2007). Clement et al, 2009 and Burgman et al, 2008 presented observational evidence for decadal changes in cloud cover, again negatively correlated with sea surface temperature in the Pacific. It appears more likely that cloud changes driven by the Pacific Ocean decadal pattern are a significant, and under appreciated, factor in global warming in the satellite era. In the 20th century in the Pacific Ocean there was a cool La Niña dominated mode from the mid 1940’s to 1976, an El Niño dominated warm mode from 1977 to 1998 and a cool mode since. That the trajectory of global surface temperature changes is a mirror of the ocean states seems unlikely to be coincidental.
The true test comes from making a falsifiable hypothesis and testing it against outcomes. The current cool mode of the Pacific decadal pattern will lead to more intense and frequent La Niña - along with cold water rising in the north-eastern Pacific in the cool mode of the Pacific Decadal Oscillation. This must result in more low level stratiform cloud cover and the planet must cool over a decade or three more. The current super La Niña is certainly leading the charge in a very significant way. With a record negative Southern Oscillation Index – this one seems like it might persist through this year at least and intensify again in the boreal spring.
References
Burgman, R. J., Clement, A. C., Mitas, C. M. , Chen, J. and Esslinger, K. (2008), Evidence for atmospheric variability over the Pacific on decadal timescales GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L01704, doi:10.1029/2007GL031830, 2008
Clement, A., Burgman, R. and Norris J (2009), Observational and Model Evidence for Positive Low-Level Cloud Feedback Science 325 (5939), 460. [DOI: 10.1126/science.1171255]
Dessler,A. E. (2010), A Determination of the Cloud Feedback from Climate Variations over the Past Decade, Science: 330 (6010), 1523-1527. [DOI:10.1126/science.1192546]
Lean, J. (2010), Cycles and trends in solar irradiance and climate, WIREs Clim
Change 2010 1 111–122, John Wiley & Sons, Ltd
Zhu, P., Hack, J., Keilh, J and Zhu, P, Bretherton, C. 2007, Climate sensitivity of tropical and subtropical marine low cloud amount to ENSO and global warming due to doubled CO2 - JGR, VOL. 112, 2007
